Borosilicate glass composition



Patented Aug. 27, 1935 2,012,552 BOROSILIGATE GLASS COMPOSITION WilliamC. Taylor, Corning, N. Y., assignor to Corning Glass Works,vCorning,'N.Y.,'a corpoj ration of New York g No Drawing. ApplicationMay 24, 1932, Serial a -No.613,339

2 Claims. (o1. roe-36.1)

'This invention relatesyto glass compositions and more particularly tocompositions for borosilicate glasses. I

The object of the invention is to produce bore-' 6 silicate glasseshaving high chemical stability and high electrical resistivity withoutproducing crystallization.

I V l V t 18 knowp that the addltlon of Second group pension ofapproximately .0000046 as calculated from the compositionusing the.Winkelmann and elements to borosilicate glassesusually results in 10increased chemical stability and althoughcertain prior borosilicateglassesvhave contained appreciable amounts of second group oxides thesevglasses have also contained a high soda alkali content. High alkalicontent as is well known tends such prior glasses because with lowalkali con.- tent the addition of any considerableamount of second groupoxides causes crystallization on working or cooling. Consequently it wasnot known how to produce 'borosilicates of low alkali content'and lowthermal expansion coefiicient which would contain large amounts ofsecond group oxides and at the same time would remain free fromcrystallization on cooling oronpbeing re-worked in the name. Low alkaliborosilicates containing large amounts of second group oxides areparticularly useful and desirable in sealing electrodes vof tungsten ormolybdenum or alloys thereof intoglass receptacles wherein chemicalstability and low electrical conductivity is required and a definite lowexpansion coeflicient is necessarytolmatch that of the metal electrode.

1. have discovered that by substituting potash for a proportion of thesoda I can preventthis undesirable crystallization and produceborosilicate glasses containing larger amounts of'sec- 0nd group oxidesin proportion to the alkali content than have heretofore been possibleand at the same time possessing the valuable properties-of high chemicalstability, low expansion coefficient and freedom from crystallization.In fact I find that I may introduce second group oxides'so that theywill constitute not less than 10% of the,

composition of the finished glass.

The following composition which meets these considerations is an exampleofa glass which I have successfully melted in accordance with myinvention. I

'crystallizing.

to decrease chemical stability and electrical resistivity. 'Considerablealkali was a 'necessityin efficient of expansion of about .000005 andconboric 0Xide8%, sodium oxide 6% and potassium The above glass has alinear coeflicient ofex- Schott expansion factors and is a clear glasswhich may be re-worked in the flame without I It also possesses goodchemical stability. v In general, other second group metals particularlyzinc,. calcium or bariummay be substituted in lieu of magnesium in theabove composition without causingcrystallization although the expansioncoefiicien't will be subject to'a'slight change depending on which metalis.

substituted. The tendency to crystallize is repressed to a greaterextent and the second group content may beincreased as the ratio NazO toK20 is decreased. Since increase of potash con- 3 tent causes the glasstobecome increasingly more difficult to melt, the ratio NazO to KzOwilldependto some extent'upon the character ofthe glass required. I havefound that very satise' factory results are obtained when this ratio isequal. to one, that is, when the amount of po t-. ash is equalf'to theamount of soda but I have also found that a ratio as low as 3Naz0 to1K2:O'30- is suitablefor many purposes as forexamplein the above recitedcomposition.

WhatIclaimis: 1. A borosilicate glass having'a thermal cosistingessentially of silica72%, magnesia 12% oxide 2%. 1 V i 2. A borosilicateglass which has a coefficient of'thermal expansion of about .000005,which'will' not crystallize upon reheatingand which consists of over %ofsilica, not over 8% of alkali of which at least one-third is potash;about 8% of boric oxide and not less than 10% 'of sec- 4 0nd groupoxides.

' 1 WILLIAM'QTAYLOR."

